Optimization of tumor laser ablation. Evaluation of thermal properties: the liver tissue case.

Cancer represents one of the main causes of death worldwide, moreover with an increasing occurrence trend. In particular, liver cancer, which frequently affects children too, represents the sixth most common tumor, with still a pretty low relative survival rate despite the research advancements in the last decades. The elective treatment for liver tumors is surgery and a lot of effort has been devoted in finding new intervention techniques that can combine high efficacy with lower patient discomfort. Among them, minimally invasive methods that induce a controlled tissue hyperthermia (radiofrequency, microwave, and laser ablations) have risen great interest because they represent a valid alternative to standard surgical resection, which is not always possible due to the peculiar characteristics of the malignant lesion or the patient general health conditions. This thesis focuses on Laser Ablation (LA), a treatment in which the cytotoxic temperature is reached exploiting the absorption of laser light delivered to the tumor site by an optical fibre applicator. Despite the many advantages of LA (for instance, the localized heating that minimizes the effects on healthy tissue and the possibility to carry out the ablation under imaging methods like the magnetic resonance), this technique is not as widely employed as other forms of ablation. This mainly because of the lack of an accurate treatment planning tool, in turn due to the difficulty to predict the outcomes given the large variability of the optical (absorption) and thermal (conductivity) parameters of the liver tissue with the location of the malignancy. The aim of this thesis project is to investigate the thermal properties of liver in view of the integration of a more realistic thermal model in a laser ablation planning tool to be used for the real-time monitoring of the LA procedure. The fundamental idea is to retrieve information on the thermal properties – mainly the thermal diffusivity – in real time by generating a small tissue heating with the laser before the proper laser ablation procedure. To this end, two methods to estimate the thermal parameters starting from the experimental temperature curves have been studied and a thermal model has been developed, solving the heat equation using the Partial Differential Equation (PDE) toolbox of MATLAB®. The procedure was experimentally validated using for simplicity ex-vivo bovine liver tissue. Clearly, using ex-vivo tissue does not allow considering all the biological phenomena, such as the impact of perfusion in the body thermoregulation; however, despite this limit, it provides a good starting point for understanding the variability of the thermal diffusivity and its dependence from the temperature. Extensive tests at different laser powers and irradiation times were performed in different ex-vivo bovine tissues. To avoid artifacts due to the interaction between the laser beam and the sensor, the temperature variations were recorded using all dielectric sensor arrays based on Fibre Bragg Gratings (FBGs). The obtained diffusivity can be used to predict different ablation procedures using a multi-physics model developed in COMSOL Multiphysics®.